1. Field of the Invention
The present invention relates to a switch-mode power supply in which a rectifier produces d.c. voltage from an input a.c. voltage and a converter is connected thereto for producing d.c. voltage.
2. Description of the Prior Art
Switch-mode power supplies are currently used on a large scale for current supply purposes as they are lighter, smaller and less expensive to produce than, for example, 50 Hz power supplies.
Generally known switch-mode power supplies consist of a transformer whose primary winding is charged with energy by a switching transistor and from whose secondary winding or windings this energy is withdrawn in the form of one or more than one constant voltage. Here, the transformer forms a part of a forward converter or a fryback converter. The construction and mode of operation of these types of converter are described in detail in the book "Schaltnetzteile" by Joachim Wustehube, published in 1979 by Expert-Verlag, Chapter 2.1.
Switch mode power supplies generally operate in a frequency range of from 16 to 500 kHz. As the occurring current curves and voltage curves exhibit considerable overshoot components, and, in addition, it is necessary to switch high currents and voltages, considerable interference must be expected in the frequency range above 10 kHz (long wave). The frequency response of this interference is subject to binding regulations if propagation into public networks or radiation into the open are possible.
For the suppression of asymmetrical interference voltages in the networks, between their primary and secondary sides transformers in switch-mode power supplies are provided with a shielding. The shielding consists of, for example, a copper foil which is arranged in an insulated manner over the primary winding.
Fryback converter power supplies contain a transformer having an air gap. In particular at high powers, high eddy current losses may occur in the transformer cores of the transformers and in the shielding. Therefore, it is general practice (see Wustehube) to use a so-called conductive fleece in place of a copper foil for shielding purposes. As a result, the production of such a transformer for a fryback converter is elaborate and expensive.
The known forward converters consist of periodically-operating switching transistors which are fundamentally controlled by a fixed timing pulse train, and a transformer without an air gap whose primary winding lies in the collector circuit of a switching transistor and from whose secondary winding a d.c. voltage can be obtained via a rectifier and inductance. Since a predetermined switching frequency is used in the case of this switching principle, the switch-on time of the switching transistor is predetermined. This switching principle has the disadvantage that a switching transistor is switched on in the presence of current and a free-wheeling diode in the secondary circuit assumes the blocked state in the presence of current. This leads to high radio interference voltages on the network line. If these converter circuits are used in text terminals of communications technology having electric isolation in accordance with the protection class 2, i.e. without ground conductors, it is not possible to shield these devices to the required radio interference level of Limit Value Class B.